The present invention relates to cementing operations. More particularly, the present invention relates to the use of a graft polymer as a fluid loss control additive in cementing operations, the graft polymer comprising a lignite backbone and grafted monomers selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylamide, acrylic acid, vinylphosphonic acid, diallyldimethylammonium chloride and salts thereof.
Well cement compositions are commonly utilized in subterranean operations, particularly subterranean well construction and remedial operations. For example, in subterranean well construction, a pipe string (e.g., casing and liners) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing operation, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the subterranean formation. Among other things, the annular sheath of set cement surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in plugging and abandonment operations and remedial cementing operations, such as squeeze cementing and the placement of cement plugs.
For such subterranean cementing operations to be successful, the cement compositions typically include a fluid loss control additive to reduce the loss of fluid, from the cement compositions, e.g., when they contact permeable subterranean formations and zones. Excessive fluid loss may cause, for example, a cement composition to become prematurely dehydrated, which limits the amount of the cement composition that can be pumped. Due to this premature dehydration, excessive pump pressure may be required to place the cement composition, potentially resulting in breakdown of the formation and/or destabilization of the well bore.
Certain polymers have been used heretofore as fluid loss control additives in cementing operations. For example, cellulosic materials such as hydroxyethyl cellulose and carboxymethylhydroxyethyl cellulose have been used as fluid loss control additives. Copolymers of 2-acrylamido-2-methylpropanesulfonic acid and acrylamide or N,N-dimethylacrylamide have also been used as fluid loss control additives. In addition, graft polymers comprising a backbone of lignin or lignite and pendant groups comprising at least one member selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, acrylonitrile and N,N-dimethylacrylamide have been used as fluid loss control additives. These fluid loss control additives, however, may not provide a desired level of fluid loss control at high temperatures (e.g., at least about 500° F. (260° C.)) and/or may be exhibit undesirable properties (e.g., dispersive).